As a data output apparatus of word processors, personal computers, facsimiles and so forth, printers capable of printing desired information such as texts and images on a sheet-type printing medium, e.g., paper, film and the like, are widely utilized.
Although various printing methods are available for such printers, recently an inkjet printing method has particularly attracted attention because of its capability to perform non-contact printing on a printing medium such as paper, ease of color printing, and low noise. Moreover, for a configuration of such a printer, in general a serial printing method is widely adopted because of its low cost and ease of downsizing. According to the serial printing method, a printhead discharging ink in accordance with desired printing data is attached to a carriage and printing is performed by reciprocally scanning the carriage in a direction crossing to the conveyance direction of the printing medium (e.g., paper).
The inkjet printing scheme is designed to print on a printing medium by discharging small ink droplets using various kinds of ink discharge methods such as an electrostatic suction scheme implemented by the application of high voltages, a piezoelectric scheme of mechanically vibrating or displacing ink (colored ink) by using piezoelectric elements, and a thermal scheme using the pressure generated when ink forms bubbles as it is heated. This scheme produces little noise during printing, and allows high resolution, high speed printing by using a printhead having ink orifices formed at a high density. Printing apparatuses using such an inkjet printing scheme are in widespread use even in homes. It has therefore become popular that photos taken by digital cameras are printed by inkjet printing apparatuses. Further, in recent years, an inkjet printing apparatus, which can print photographs by directly connecting with a digital camera, so as not to use PCs when printing photographs taken by the digital camera has been proposed.
As digital cameras have been in widespread use, strong demands have arisen for more inexpensive, higher performance inkjet printing apparatuses.
An inkjet printing apparatus is designed to print by discharging liquid ink toward a printing medium through small holes (nozzles) formed in the printhead. It is, however, known that since ink is a liquid, when the nozzles are exposed to the atmosphere, the ink in the nozzles increases in viscosity and solidifies.
As ink increases in viscosity and solidifies, a discharge failure, e.g., the occurrence of the landing position offset of an ink droplet or a non-discharge state in which no ink droplet is discharged, occurs, resulting in a deterioration in the quality of a printed image. In order to prevent this, the inkjet printing apparatus has a recovery mechanism for setting the apparatus in a good discharge state. This mechanism performs suction recovery operation of producing negative pressure in the printhead by suction or pressurization, thereby discharging ink in the printhead, or preliminary discharge operation of discharging ink irrespective of printing. Such recovery operation is performed when a predetermined period of time has elapsed while the nozzles are exposed to the atmosphere. In this operation, ink which has increased in viscosity and solidified is discharged outside the nozzles.
In general, preliminary discharge is performed at a predetermined position, e.g., a cap which is provided near the home position and also used for suction recovery or a preliminary discharge port provided on the opposite side of the printing area to the home position.
Caps for suction recovery are indispensable for inkjet printing apparatuses, and hence printing apparatuses have them. In contrast, a preliminary discharge port is preferably provided with a member which absorbs ink, and is not used for anything other than preliminary discharge, and hence some printing apparatuses are designed without a preliminary discharge port in consideration of the cost and space required for the printing apparatus. With this arrangement, preliminary discharge is performed only at the preliminary discharge port.
In addition, since a cap has a mechanism for suction recovery, ink (including pigment ink) discharged by preliminary discharge can be discharged outside the cap. In contrast, a preliminary discharge port often has no suction mechanism. If, therefore, pigment ink is preliminarily discharged through the preliminary discharge port, the pigment ink solidifies into stalactite-like clusters and is deposited. As the degree of deposition increases, the deposit comes into contact with the discharge surface of the printhead or the like. This damages the discharge surface or the like.
For the above reason, a printing apparatus which discharges pigment ink is designed to mainly perform preliminary discharge only at the cap.
Since preliminary discharge is performed independently of normal printing operation, it takes time in addition to the time for printing. For this reason, as the number of times of preliminary discharge increases, the throughput decreases. As described above, preliminary discharge has great influence on the throughput.
In general, however, preliminary discharge intervals are determined by the ink and printhead to be used for printhead. If, therefore, the preliminary discharge intervals are simply prolonged, a discharge failure tends to occur due to an increase in viscosity of ink and its solidification.
Under the circumstances, there have been proposed preliminary discharge control methods which improve throughput by optimizing control and eliminating wasteful operation.
For example, in a printing apparatus having, as positions where preliminary discharge is to be performed, two positions, i.e., the position of a cap provided near the home position and a preliminary discharge port provided on the opposite side to the cap within the moving area of a carriage, when a signal for designating preliminary discharge operation is received, the time required to transfer print data to be printed by the next scanning operation is compared with the time required for movement to a position where preliminary discharge can be done next, and control is performed to select one of the preliminary discharge positions which corresponds to a shorter time (Japanese Patent Laid-Open No. 2003-136755).
In that control method, however, since two times like those described above are calculated, compared, and selected for each scanning operation, the load on a controller (a control unit including a CPU) which performs processing increases.
It is therefore necessary to use high-performance, expensive components with high throughput for the controller, resulting in an increase in the cost of the apparatus. In addition, if the above control is applied to an inkjet printing apparatus having a control with low processing throughput, a desired throughput cannot be obtained because of a decrease in processing speed.